In a radio communication system, a mobile terminal communicates with another mobile terminal via a base station in a cell to which the mobile terminal belongs. In an environment of radio communication, the mobile terminal receives not only a signal transmitted from the base station in the cell to which the mobile terminal belongs but also signals transmitted from a base station in a neighboring cell, but the signals from the base station in the neighboring cell are not communication signals that the mobile terminal desires to receive and these signals constitute interference signals for the mobile terminal. That is, the mobile terminal receives inter-cell interference in the communication process. When the mobile terminal is located at a cell edge, inter-cell interference becomes stronger. This also constitutes a main factor for limiting the throughput of the signal. Therefore, the suppression of inter-cell interference is an important challenge to the radio communication system.
To remove inter-cell interference, a prior art performs communication according to a scheme of coordination among a plurality of base stations and effectively suppresses inter-cell interference. According to the scheme of coordination among a plurality of base stations, a plurality of antennas are arranged in the base station and directivities of the antennas can be changed by a precoding vector of an antenna array. When a plurality of base stations coordinate, channel information of the mobile terminal is transmitted to base stations causing interference with the user via a backhaul link. In such a case, the base stations can avoid interference with the mobile terminal in question by setting the precoding vector and also enhance the link with mobile terminals which belong thereto.
FIG. 1 shows an image illustrating how multiple antennas of a plurality of base stations used in the prior art remove inter-cell interference. As shown in FIG. 1, in the multiantenna system with the plurality of base stations, base station 1 (eNB 1) communicates with mobile terminal 1 (UE 1) via a related link and base station 2 (eNB 2) communicates with mobile terminal 2 (UE 2) via a related link. When mobile terminal 1 and mobile terminal 2 are located at a cell edge, in a downlink direction from the base station to the mobile terminal, a signal transmitted from base station 1 to mobile terminal 1 in the subject cell causes interference with mobile terminal 2 in the neighboring cell via an interference channel. Similarly, a signal transmitted from base station 2 to mobile terminal 2 in the subject cell also causes interference with mobile terminal 1 in the neighboring cell via an interference channel. To remove inter-cell interference, the backhaul of base station 1 and base station 2 can exchange information between the backhaul of base station 1 and base station 2 by making connections according to a wired scheme or wireless scheme such as optical fiber and channel. Through information exchange between the base stations, multiple antennas are arrayed in the respective base stations through precoding so as to prevent the antennas of base station 1 from being directed to mobile terminal 2 in the neighboring cell and prevent the antennas of base station 2 from being directed to mobile terminal 1 in the neighboring cell, and it is thereby possible to avoid inter-cell interference.
The conventional method of setting a precoding vector is based on optimization for a signal to interference noise ratio (SINR). FIG. 2 shows an image of calculating inter-cell interference. In FIG. 2, base station 1 (eNB 1) communicates with mobile terminal 1 (UE 1) via communication link 11 and base station 2 (eNB 2) communicates with mobile terminal 2 (UE 2) via communication link 22. Furthermore, a signal transmitted from base station 1 to mobile terminal 1 in the subject cell causes interference with mobile terminal 2 in the neighboring cell via interference link 12. Similarly, a signal transmitted from base station 2 to mobile terminal 2 in the subject cell also causes interference with mobile terminal 1 in the neighboring cell via interference link 21. To reduce inter-cell interference, it is desirable to intensify precoding vectors in the directions of link 11 and link 22 and reduce precoding vectors in the directions of link 12 and link 21.
To set such precoding vectors, the prior art optimizes the SINR and thereby sets precoding vectors capable of reducing inter-cell interference. Assuming that the base station side has a plurality of antennas and the user side has one antenna, the SINR can be expressed by following equation 1.
                              (                      Equation            ⁢                                                  ⁢            1                    )                ⁢                                                                                                            SINR            1                    =                                                    link                ⁢                                                                  ⁢                11                                            noise                +                                  link                  ⁢                                                                          ⁢                  21                                                      =                                                                                                                                                                            h                          →                                                11                        H                                            ·                                                                        w                          →                                                1                                                                                                  2                                ·                                  P                  1                                                                              σ                  2                                +                                                                                                                                                                                h                            →                                                    21                          H                                                ·                                                                              w                            →                                                    2                                                                                                            2                                    ·                                      P                    2                                                                                      ,                            [        1        ]                        and                                                                SINR          2                =                                            link              ⁢                                                          ⁢              22                                      noise              +                              link                ⁢                                                                  ⁢                12                                              =                                                                                                                                                          h                        →                                            22                      H                                        ·                                                                  w                        →                                            2                                                                                        2                            ·                              P                2                                                                    σ                2                            +                                                                                                                                                                h                          →                                                12                        H                                            ·                                                                        w                          →                                                1                                                                                                  2                                ·                                  P                  1                                                                                                    
Here, SINR1 and SINR2 represent SINRs of user 1 and user 2 respectively, P1 and P2 represent transmission power of base station 1 and base station 2 respectively, σ2 represents noise power of the user, w1→ and w2→ represent precoding vectors of base station 1 and base station 2 respectively and hmn→ represents a channel vector from base station m to user n (m=1, 2, n=1, 2).
By optimizing equation 1 above, it is possible to find out the amount of precoding that maximizes the sum of rates of two user stations using equation 2 below.
                              (                      Equation            ⁢                                                  ⁢            2                    )                ⁢                                                                                                                          arg              ⁢                                                          ⁢              max                                                                        w                  →                                1                            ,                                                w                  →                                2                                              ⁢                                          ⁢                      log            ⁡                          (                              1                +                                  SINR                  1                                            )                                      +                  log          ⁡                      (                          1              +                              SINR                2                                      )                                              [        2        ]            
Finding out the amount of precoding that maximizes the sum of rates of the two user stations makes it possible to effectively improve the system throughput.